Currently, an alternating-current (AC) discharge lamp with a reflective mirror (hereinafter, referred to as a lamp), particularly when combined with an elliptical reflective mirror, has an increase in the temperature of the electrode on the opening part side of the elliptical reflective mirror due to the reflected light from the optical system. Consequently, a temperature difference develops between the two electrodes, and the normal halogen cycle no longer functions. As a result, sometimes, the electrode tip on the opening part side of the elliptical reflective mirror erodes, and the lamp characteristics cannot be maintained. In addition, the electrode shape deforms and an offset of the arc spot is produced by the erosion of the electrode. Generally, for an AC high-pressure mercury lamp, spot offset in each cycle gives the impression of “flickering.”
As a remedy, PCT Application PCT/IB95/00392 shows a method that adds a pulse superimposed on the current waveform in each cycle, increases the temperature of the electrode tip, and optimizes the halogen cycle. However, in the proposed method, a constant current pulse is always generated, and the halogen cycle is optimized, conversely, substantial damage to the electrode is possible.